Email Aliases, or Email forwarders, allow you to forward mail intended for one address to another. For instance, if you are jon@anydomain.com and you want sales@anydomain.com to be sent to you - set up an alias. Or if your webmaster is an offsite contractor, you can set up webmaster@anydomain.com to forward to their email address (e.g. - sara@webmasters.com).
Every account also has a master "catch-all"" email address that gets all of the mail associated to that domain that has an unspecified address.